1. Field of the Invention
The present invention relates to optical waveguide devices. In particular, the invention relates to a device where active elements, such as a semiconductor laser and semiconductor optical modulator, are monolithically integrated, an optical transmission module using such devices and an optical communication system unit using such modules.
2. Description of the Related Art
Optical waveguide devices, such as semiconductor lasers, optical modulators, optical amplifiers and optical detectors, are key devices used for optical communication, optical data storage, measurement and the like. With the recent advance of optical components in miniaturization, monolithic integration of such individual devices is becoming more popular. One typical example is an optical communication chip where a semiconductor laser and an optical modulator are monolithically integrated.
In a typical light emitting or amplifying waveguide device such as a semiconductor laser or optical amplifier, its active region has a hetero junction consisting of a p-type semiconductor and a n-type semiconductor. To this p-n junction, a voltage is applied so that the p-type semiconductor side has a higher potential while the n-type semiconductor side has a lower potential. This injects carriers into the junction constituting the active region, resulting in light emission and amplification. In the case of such a device as a semiconductor optical modulator or optical detector, although it has a similar p-n junction, this p-n junction is biased to absorb light in such a manner that the n-type semiconductor has a higher potential while the p-type semiconductor has a lower potential. Hereinafter, biasing from the p-type semiconductor to the n-type semiconductor is called forward bias whereas biasing from the n-type semiconductor to the p-type semiconductor is called reverse bias.
A semiconductor optical module device is a device where two or more abovementioned optical waveguide devices are integrated. Therefore, it is sometimes necessary to integrate devices which must be biased in the mutually opposite directions. As described earlier, typically, a semiconductor optical waveguide device has a p-n junction. By using the MBE method, MOCVD method or the like, this p-n junction is formed by epitaxially growing an n-type semiconductor and p-type semiconductor on a semiconductor substrate. In the case of a GaAs or InP substrate, the substrate has a thickness of about 100 μm or more. By contrast, epitaxially grown semiconductor layers are at most of an order of 10 μm in thickness. Accordingly, even if devices which are biased in the mutually opposite directions are integrated in a chip, the substrate side of either device is usually set to the same potential when the chip is used. This is because the substrate side of one device is electrically connected with that of the other device since the substrate side of either device is of the same conductivity type and grown epitaxially on the same substrate.
One example is an EA/DFB chip used as a light source for optical fiber communication. It contains a distributed feedback (DFB) laser and an electroabsorption (EA) modulator. When the DFB laser is forward biased, the DFB laser lases to continuously emit light which enters the EA modulator via an optical waveguide formed in the chip. The EA modulator is made of a semiconductor whose bandgap wavelength is shorter than the wavelength of the incident light from the laser. When no voltage is applied to between the anode and cathode of the EA modulator, the EA modulator is transparent for the laser light. Propagating through the waveguide of the EA modulator, the laser light incident on the modulator is emitted from the front of the chip. If the EA modulator is reversely biased, the bandgap wavelength becomes longer due to the quantum confined Stark effect. The EA modulator becomes not transparent for the incident laser light. Since the light is absorbed by the EA modulator, no light is emitted from the front. To perform modulation, the EA modulator repeatedly switches on and off the laser light at high speed by alternately serving as an optical transmitter and an optical absorber.
Today, the EA/DB laser is fabricated as below. Typically, an EA modulator and a DFB laser, both multi-layered, are epitaxially grown on an n-InP substrate which is an n-type doped semiconductor. The EA modulator and the DFB laser may be grown either concurrently or separately. After the epitaxial growth, the top p-type semiconductor is shaped by photolithography technology and etching into a stripe of several μm in width to form an optical waveguide. Usually, the same tripe is used by both DFB laser and EA modulator so that they will optically be coupled to each other. As the case may be, not only the p-type semiconductor layer on the top of the stack structure but also the n-type semiconductor below the p-n junction are etched into a stripe. In the case of a buried type chip, the semiconductor-removed etched regions are filled again with a semi-insulative semiconductor.
The following describes how the EA/DFB is driven. In order to operate the two integrated devices independent of each other, it is necessary at least to electrically separate the p or n side electrode of one device from that of the other device. Usually, an isolation region is formed between two devices. Since the upper cladding layer is shaped into a several μm width stripe, the isolation region can be made enough long relative to the small section area to impose a large separating resistance between the two devices. In the case of the aforementioned EA/DFB, the top anode side, that is, the p side is separated with a separating resistance in the range of several ten kilo-ohms to several mega-ohms.
If the isolation region cannot be made enough long, low resistance layers such as the electrode contact layer in this region are removed by etching. Alternatively, ions are implanted into this area to enlarge the resistance.
Whereas the p side electrode (anode) of one device is electrically separated from that of the other device as described above, the n side electrode (cathode) of either device is formed on the same substrate used as a common ground with no electrical separation between the integrated devices.
This EA/DFB chip of the conventional structure has the common ground. To drive the integrated devices independently of each other by biasing them in the mutually opposite directions, it is therefore necessary for each device to have a separate drive power supply. In the EA/DFB, the DFB laser is driven by a positive power supply whereas the EA is driven by a negative power supply.
FIG. 1 is a simplified circuit diagram for showing how the conventional EA/DFB is driven. The DFB laser is driven by applying a positive voltage to the anode and grounding the cathode. This causes an electric field from the higher potential anode to the lower potential cathode, which implants carriers into the active layer. Meanwhile, the electroabsorption optical modulator is driven by applying a negative voltage to the anode and grounding the cathode shared by the DFB laser. This causes an electric field from the higher potential cathode to the lower potential anode, which changes the bandgap to absorb light. That is, the DFB laser is driven by a positive power supply whereas the EA modulator is driven by a negative power supply.
FIG. 3 shows a conventional ridge waveguide EA/DFB chip. FIG. 4 shows a cross-sectional view of the active layer of FIG. 3 taken along the direction of the waveguide. FIG. 5 shows a cross-sectional view of the EA modulator taken along the direction perpendicular to the waveguide. The DFB laser and the EA modulator are integrated on a n-type InP substrate 101. Although the DFB laser and the EA modulator are electrically separated in the isolation region, they are optically coupled by the optical waveguide there. This isolating region is made enough long in the waveguide direction. Alternatively, either the highly doped top layer formed to provide ohmic contact with the electrode metal is trimmed by etching or ions are implanted 112 so as to sufficiently raise the isolation resistance. This resistance suppresses electrical crosstalk between the devices, allowing stable operation. Also note that the n-side electrode 113 is formed as a common electrode below the n-type InP substrate 101. This n side electrode is to be grounded.
In this conventionally structured EA/DFB, since a common ground is shared by the respective integrated devices, the p electrode 109 of one device must be set to a positive potential so as to forward bias the device whereas the p electrode 107 of the other device must be set to a negative potential so as to reversely bias the device. That is, such an integrated device structure requires one positive power supply and one negative power supply at least. The same holds for integration on a p-type substrate. In this case, an anode p electrode 113 is formed below the substrate and used as a common ground.
Although a device to be biased forward and a device to be biased reversely are integrated in such a chip as an EA/DFB, it is rational to drive them by a single power supply since the configuration can be miniaturized and simplified due to the elements decreased.
To give a positive bias to one device and a negative bias to the other device by a signal positive or negative power supply, such a circuit as shown in FIG. 2 must be configured. In this configuration, if the two devices are monolithically integrated, both anode and cathode of one device must be electrically separated from those of the other device whereas in the conventional configuration, only anode or cathode of one device must be electrically separated from that of the other device.
If an electrode is formed as a common ground at the bottom of a substrate as in a conventional EA/DFB, it is structurally difficult to electrically separate the ground side. To realize separate ground side electrodes in a semiconductor optical waveguide chip, structural invention is necessary.
Among the prior inventions concerning integrated device structures to be driven by signal power supplies, there are Japanese Patent Laid-open No. 9-51142 and Japanese Patent Laid-open No. 2000-232252. Japanese Patent Laid-open No. 9-51142 discloses a structure characterized in that the p type and n type semiconductor layers of one device are epitaxially grown in this order on a conductive substrate whereas the n type and p type semiconductor layers of the other device are grown likewise in this opposite. These devices are driven by giving the same bias from the top to bottom by using a single power supply. In this structure, the device properties may deteriorate since many p-n junctions are formed. In addition, this structure is not feasible since it is difficult to epitaxially grow such layers. Japanese Patent Laid-open No. 2000-232252 discloses a structure characterized in that a layer which exists between the substrate and an active layer of a device is oxidized to electrically separate the device from the substrate. Since the layer to be oxidized in this structure must contain aluminum, its applicability depends on the material. In addition, this structure has a drawback in which the coupling efficiency between the devices is low since the devices are not coupled by the waveguide.